This invention relates to delayed coking, and more particularly to an improvement in coker furnaces associated with delayed coking units.
In the delayed coking process, a petroleum residuum is heated to coking temperature in a coker furnace, and the heated residuum is then passed to a coking drum where it decomposes into volatile components and delayed coke. The delayed coking process has been used for several decades, primarily as a means of producing useful products from the low value residuum of a petroleum refining operation.
Coker furnaces typically include multiple banks of heater tubes wherein each bank is comprised of a series of straight sections connected by return bend elbow fittings. During the operation of the coker unit, in which the coker feedstock is heated to temperatures of 900.degree. F. or more, the furnace tubes become fouled by coke deposition on the interior surface of the tubes. As this fouling process progresses, the furnace efficiency drops, and progressively more severe furnace conditions are required to heat the incoming feed to coking temperature. As a result of this internal furnace tube fouling, it is necessary to periodically decoke the furnace tubes.
There are several methods used to decoke the furnace tubes. In some procedures, the furnace is taken out of service during the decoking procedure. In other procedures, only a part of the tube banks are removed from service. In all cases, production is either halted or reduced during the furnace decoking process.
One decoking procedure, sometimes referred to as on-line spalling, involves injecting high velocity steam and cycling the furnace tube temperature enough, such as between 1000.degree. F. and 1300.degree. F., to cause contraction and expansion of the tube, with resultant flaking off of the accumulated coke deposits, which deposits are then blown from the furnace tubes by steam flow. This procedure can be carried out on a portion of the tube banks while another portion of the tube banks remains in production.
Another decoking procedure involves injection of air along with the steam at some stage of the decoking. Because the tubes are still very hot during the decoking, the air combusts the coke deposits, such that there is a combined spalling and combustion of coke.
The above-described decoking procedures, including variations thereof, are well understood by those in the coking industry.
A common problem in decoking is that coke particles removed by the decoking process cause erosion of the furnace tubing, particularly at the return bend elbow fittings connecting adjacent straight sections of furnace tubing.
In the past, the erosion problem has been addressed in a number of ways, including using an erosion resistant metal composition, using very thick-walled piping, and in some cases by adding a weld overlay to the most erosion-prone sections of the piping.
In U.S. Pat. Nos. 4,389,439 and 4,826,401 to Clark, a technique for improving the erosion resistance of metal surfaces is described. The technique includes a boron diffusion step to improve the erosion resistance of metal piping.